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Can all helicopters hover, and how long does it take to "stop"?
Following the accident of Kobe Bryant's helicopter N72EX, there are a few things that puzzle me:

While waiting for clearance to fly close to Burbank airport, the helicopter didn't "stay in place", but actually made all sorts of loops and circles over Glendale:

Source: Flightradar
Does this mean that such an helicopter can't actually "hover", and has to move forward to stay up in the air? There are definitely helicopters which seem to be able to stay in the same location (TV helicopters, police helicopters), so I wonder what is the difference (size? load?). Or is it just "cheaper" to make loops rather than hover?
This question and associated answer seem to indicate that there is actually a limit to the "hovering" capability of helicopters "in the air", though I'm not what the consequences are when you go beyond the limit. Is the helicopter just not able to "stay in the air"? Or does it need to move forward to be able to maintain level flight? How does it actually work, shouldn't "tilting" the helicopter forward reduce the "upwards" force?
At the end of this ill-fated flight, the helicopter makes a left turn until it crashes in the mountain.

Current "expert" analysis (pending NTSB's inquiry) point to an issue with the weather that suddenly got very bad (basically the helicopter being sandwiched between very low ceiling and rising terrain) which made the pilot make a manoeuvre that ended up being a very bad choice.
Could the helicopter not just have "stopped" or at least "slowed down" rather than turn? If it should have been able to stop, how long (in time or distance) would it have taken?

When you're in a queue at the bank, grocery store, whatever, do you stand on one leg, blindfolded, just for the challenge?  Hovering a helicopter is a bit like that.

@J... You forgot "on top of a balance board."

@J... It seemed so much easier than that, but evidently it's much more difficult than it looks like!

Certainly not a dupe, but this is related.

@J... Well if the choice is between that and walking into the wall of the bank...

As noted in another answer, all helicopters can hover, but a so-called "high hover" (out of ground effect or especially at operational altitude) is a more difficult maneuver, requiring more power than a ground hover, and being harder to maintain (because reference points are much further away).
Helicopters generate more lift for the same power when in forward flight, and transitioning from forward to hover requires a well-controlled combination of adding power, adjusting collective, and maneuvering cyclic (in both axes) and anti-rotation controls -- which is to say it's much harder than simply flying forward at low speed.  In general, once aloft and flying forward, it's much easier to continue forward flight (it's also safer, since in the case of a failure, autorotation works better if you already have some forward speed).
Hovering on instruments is even more difficult than a high hover -- so if visibility is a problem, it's generally far better to continue forward flight than to attempt a high hover.

That must be the point I don't quite get: "Helicopters generate more lift for the same power when in forward flight". My understanding is that to move forward, the helicopter is somewhat "tilted forward" (nose down), and that what was vertical lift is now split into a (smaller) upwards component and a forward component. How is more lift generated in this situation? Is it the airflow from the forward motion that "adds" to the airflow from the rotation motion and thus generates more lift?

The way I've seen it diagrammed, "translational lift" is due to the increase in airflow through the rotor disc compared to hover.  Unlike an autogyro, this airflow is downward from above (because the collective is positive, where an autogyro has a negative blade pitch to ensure it autorotates), and the more air is added to the hover condition, the more lift is generated (or the less power is needed for the same lift).

A hovering helicopter operates in its own downwash, which reduces lift an requires more power. Helicopters in mountain rescue operations often crash when they come into a high-altitude hover but lack the power to maintain it. (@jcaron)

@RainerP. That certainly contributes.  As I recall, both factors (airflow direction/volume in translation vs. hover and downwash) contribute.

@jcaron another (simplified) way of looking at it is that the rotor disk behaves like a wing: it does not allow the horizontal airflow from forward flight though itself (in powered flight at least), so it is deflected and creates additional lift, just like it would if it met a wing in the shape of the disk. Of course it is a low aspect ratio and inefficient wing, but it works nonetheless.

technically, rotor blades have a much higher aspect ratio than almost any wing (besides possibly those of gliders).

@Skyler In forward flight, however, the rotor disc is treated as a single lifting surface, not two or more racing around a pivot point -- and that single surface has a 1:1 aspect ratio, lower than virtually any fixed wing aircraft other than the Flying Flapjack.

@jcaron The airflow over the body of a helicopter generates lift with forward airspeed, just like an airplane. It's not as large of a percentage of the lift as with an airplane, of course, but it's still quite significant.

Yes all helicopters can hover, but it requires:

More concentration to hover than to fly, because helicopters are unstable in the hover in pitch and roll. Forward airspeed provides stability and flying a helicopter with forward airspeed is comparable with flying a fixed wing plane, while hovering is comparable with standing on top of a large inflatable ball.
More power to hover than to fly with forward airspeed. This is because in the hover, there is more induced drag than in forward flight. The graph below is from the linked answer and shows the dip in total required power as airspeed increases from zero.

In order to hover, available power must be larger than required power. Available engine power reduces with increasing altitude due to the decreasing air density, and this results in helicopters having a hover ceiling, where power available is equal to power required.
Ground effect reduces required power, resulting in two hover ceilings, In Ground Effect and Outside Ground Effect. But even below the OGE hover ceiling, it is simply safer for a helicopter to pick up forward airspeed shortly after takeoff:

As stated, flying at speed leaves more fuel on board for the required trip duration.
While hovering OGE, altitude must be kept using the altimeter, while unstable pitch and roll must be corrected. The concentration required during looking at the instruments reduces situational awareness. Keeping the helicopter level cannot be done using only the instruments since the peripheral vision is not involved. Where the wind takes the helicopter cannot be viewed from the instruments, and onflying fixed wing aircraft are much harder to focus on.
If altitude is not maintained in OGE hover, there is the possibility that the helicopter enters vortex ring state, a dangerous situation where it sinks into its own rotor wake. Vortex ring state does not exist when flying forwards.

I really don't understand why vortex ring state doesn't occur at all times during 0 IAS operation, and I suspect it partially does.  I realize I have 20/20 hindsight here, but having it occur inside a walled heliport (e.g. Bin laden's place) seems completely obvious to me.

@Harper-ReinstateMonica The lower side of the rotor disk pushes air down, the upper side sucks air in. To an extent, even with no sink rate, there is some reverse flow at the blade tips which is noticeable as a slight loss of lift and increase of drag. The tip effect is normally accounted for when dimensioning engines and rotors, which all goes down the drain if the helicopter sinks into its own downwash.

"hovering is comparable with standing on top of a large inflatable ball" - Why are there not autonomous systems for maintaining the hover?

@aroth auto-hover systems exist, but are not very widespread in civilian designs due to the small market, which causes a rather slow adoption of new technology.

Isn't forward motion more akin to doing a rail-slide than flying a fixed wing craft ? :-)

@aroth Larger helicopters indeed have Stability Augmentation Systems that provide auto stabilisation of the hover. They’re expensive...

@RussellMcMahon I've heard it described by a licensed fixed-wing pilot taking an early helicopter transition lesson as "like flying an out-of-trim Cessna".  The faster your forward speed, the more roll you have to correct (unbalanced lift between advancing and retreating blades), the more power you apply the more tail rotor needed, and there's still the constant juggling of throttle and collective to manage altitude and RPM -- but it's still easier than hovering.

When navigating WX in a helicopter, it is often less workload to fly orbits.  It gives additional perspective and permits easy lateral movement during the course of the orbit.  It also reduces configuration changes, and possible power changes, as the aircraft can be kept in translational lift.
IFR hold navigation for helicopters is essentially identical to airplanes, and regular holds on an intersection or navaid are assigned by ATC. This is for informational purposes relative to the OP question, because in the example the helicopter was VFR or SVFR and a conventional instrument hold would not be used unless an IFR clearance was issued.  Hovering in one spot with no visual reference cannot be readily accomplished.  Most helicopters are not instrumented for a totally IMC hold.  For example, small motion inline with the longitudinal cannot be accurately determined with the instruments normally used for IFR flight.  While GPS/IMS/FMS could provide that information, it is not conventionally done.  In short, a helicopter flies IFR like an airplane and hovers with visual reference.
In summary, an orbit allows better visibility in all directions and therefore better situational awareness, and it does not require a configuration change, and it requires less power per unit time if the airspeed is in a reasonable envelope.

Just a side note, as it does not directly address the OP question...In the Vietnam Conflict, it was common practice to try to select a LZ which was a bit larger, so that the helicopter could orbit in the immediate vicinity of the LZ and with more protection from enemy fire.  The lift off of a (too) heavily loaded evacuation flight would be in ground effect and transitioned skillfully to forward motion, so that the translational lift would assist the climb.  Of course, pilots are taught to never overload their helicopters...

Regarding Vietnam and helicopters in ground effect, I highly recommend the book "To The Limit". One thing I remmeber from that book: helicopters were woefully underpowered for the tasks they were used for!

@sandos re Vietnam and choppers - I very strongly recommend "Chickenhawk". Funnier than "M.A.S.H." Bloodier, scarier, more sobering, more enlightening more real and more interesting than "M.A.S.H." A real world account of the utter terror of flying a UH-1 "Huey" helicopter  (NOT a Cobra) in Vietnam.

I have several aging buddies who were Huey pilots, and told tall tales of being way overloaded, and unable to leave the LZ unless in translational lift.  Later, I learned how that might be so, and then realized just how overloaded they must have been. When confronted, they denied ever being overloaded, just a hot, humid day.

When a helicopter hovers, it's basically is sitting in its own wash. By pushing air down, it creates a low pressure region above itself, and a high pressure area below it. To stay hovering, it has to draw air from the low pressure area and push it into the high pressure are below it, which takes a lot of energy. If it instead flies forward, it encounters fresh air without (as much of) a pressure differential to fight.
The math on it: Suppose you have a helicopter with mass $m_1$ staying in the air for time $t$. If it were just in free fall, it would acquire a velocity of $gt$, for a momentum of $m_1gt$. So for it to not acquire any downward velocity, it must somehow shed $m_1gt$ of momentum. So it needs some reaction mass to transfer that momentum to. That mass is air. If it pushes air with mass $m_2$ downwards at velocity $v_2$ (i.e. wash speed), the momentum will be $m_2v_2$. Setting $m_1gt$ equal to $m_2v_2$, we find that $v_2 = \frac{m_1gt}{m_2}$. The energy of this air will be $\frac{m_2v_2^2}2$, or $\frac{m_2}2(\frac{m_1gt}{m_2})^2$, which reduces to $\frac{(m_1gt)^2}{2m_2}$.
So the more air the helicopter pushes down, the lower the wash speed is, and the less energy the helicopter uses. By continuing to fly forward rather than hovering, the helicopter encounters more air, allowing a lower wash speed.
This is a phenomenon for all heavier-than-air aircraft: the faster they fly, the easier it is to produce lift.

Question 1: No, an S-76 can hover, but it's more energy intensive than economy cruise. It made more sense to circle and hold in an area as opposed to hovering. In addition, hovering at altitude can be hazardous in the event of an engine or tail rotor failure and having some forward airspeed can aid in making an autorotative landing, if needed.
Question 2: What brought about the accident is speculative until the NTSB releases its report.  We won’t get a definitive answer until then.  We do know that the entire Los Angeles basin was reporting low overcast and both KBUR and KVNY were reporting IFR weather condition locally.  The helicopter holds clear of KBUR Class C until it receives a special VFR clearance, then heads northwest, skirting around the edges of the KVNY Class D surface area, turning left to head south toward Calabasas and following CA101 through the canyons at a high rate of speed, around 120 KIAS.  One of the last ATC interactions was to tell SoCal approach that he was maneuvering to avoid clouds.  Exactly what factors led to the accident at that point is unknown, though as a pilot I have a few theories.  It appears as though the pilot was flying in SVFR, but with helicopters, the visibility can be as low as 1/2 mile (800 m) for SVFR operations.  Being in a narrow canyon with marginal weather conditions and attempting to fly at a high rate of speed, there probably weren’t very many options left if the canyon became socked in.
A fully loaded S-76 weighs in at around 11,000 lbs (5,000 kg) and cruising at 130 KIAS it's going to take quite a bit to stop it.  That may be beyond what the visibility that day would allow for.

Removed another speculation from your answer. Please do not speculate on ongoing investigations.

Not all can hover indefinitely.
A fully loaded Soviet era MI-24 Hind attack helicopter could only hover for 15-20 seconds, before the engines are damaged from the overload.
US intelligence agencies wondered why the Hinds always seemed to make a running takeoff rather than pull up in a hover to take off, until they got their hands on one and found that out.

However, it would be very wrong to assume that Mi 24 is uncapable of such maneuvers or even more demanding low-velocity maneuvers. The stress should be on "fully loaded" here https://youtu.be/waHOJ5LaEvc?t=161 The transport compare is big in this bird and it certainly is not going to be fully loaded with transported personnel and their equipment in an attack role.

As many of the other commenters here mentioned, it is much easier and lower work load for the pilot to fly forward than hover. It also takes more power from the engine to hover than it does to fly forward, and this largely has to do with the effects mentioned above about having to pull air from above the rotor to below. That also saves me quite a bit of gas. One thing I do not see mentioned here is that it is also significantly safer to fly forward than hover, in the sense that there is a much larger margin for error in forward flight than there is in a hover, and I'm going to make an effort to explain.
Helicopters, in the case of engine failure, can auto-rotate. This basically means you're "gliding" the helicopter (it works like those sticks with a propeller on them that you spin between your hands and they fly a little bit). Auto-rotating while moving forward is much easier to do, and is much more gentle, than auto-rotating in a hover. If I auto-rotate in a hover, I first have to convert some altitude into moving forward, and once I'm doing that, I can "glide" towards the ground. When I get near the ground, I convert the forward speed I now have into the rotor, so I basically come to a stop and a gentle landing. What does this mean, in practice? It means that if I'm moving forward, I can auto-rotate at any altitude. I can auto-rotate anywhere from 20ft to 15000ft altitude. If I'm in a hover and have to auto-rotate, I will probably need somewhere between 200-500 feet of altitude to successfully auto-rotate and land safely. When I'm flying, I'm comfortable hovering very near the ground (1-30ft) and I'm comfortable hovering at 500ft+. I'm far less comfortable (from a safety perspective) hovering at 250ft than I am at 1000ft.
TLDR;
I tend to think of the rotor spinning as a "battery". If the rotor stops spinning, I have no energy, and I will fall out of the sky. The rotor is constantly using energy to keep me flying, and that use of the rotor's energy will slow it down. I can add more energy into the rotor by using the engine, but I can also convert both forward movement and altitude into spinning the rotor. If I lose the engine, I will start to descend to keep the rotor spinning until I get near the ground. Now there's no free lunch, so what I can't do is trade my altitude for rotor speed, and then use exactly the same energy to trade my rotor speed into stopping my fall. It has friction, etc., and so I will hit the ground VERY hard if I do that. What I can do though, is be moving forward too! So now I'm moving forward, and I'm trading altitude for rotor speed, which basically means I'm falling slowly (descending). When I get near the ground, I can slow the helicopter's forward movement, and convert the forward energy into rotor speed too! This means I can stop moving forward, and as a result, have a very soft landing. That is why I can't auto-rotate from a hover easily. First, I have to convert some of my altitude into moving forward, and only then slow down my descent as I get near the ground. Then I convert that forward movement into making my landing nice and gentle. Converting that altitude into forward movement will take me about 200-400ft, which is why helicopter pilots don't like hovering at low altitudes.

END TLDR;

All helicopters can hover. That's the key advantage of that type of aircraft over the autogyro. Almost all rotating-wing aircraft are today helicopters, with only a relatively small number of autogyros still around. The most advanced autogyros of the 30s, before helicopters existed, were capable of vertical take-off and landing, but could not hover.

Not all helicopters can hover out of ground effect at all weights.  The max weight to take off & operate safely may be well above the max weight to hover out of ground effect.  Hovering in ground effect takes considerably less power than hovering out of ground effect.

In ground effect or not, hovering is a distinctive feature of helicopters. No gyroplane can hover, even in ground effect...

Some interesting information that I was told by a very experienced helicopter pilot today... things that aren't obvious...
First, understand that a helicopter in hover has zero natural stability. Unless the pilot maintains active and immediate control over it, as in using visual references, a helicopter in hover will begin to change attitude and velocity and accelerate those changes until it crashes. In forward flight mode, the helicopter has the natural stability of an aircraft.
At low airspeeds, aircraft will warn the pilot with a stall warning horn that flight characteristics are about to drastically change. Helicopters do not inform the pilot when the helicopter is transitioning from forward flight into hover. It's up to the pilot to know this.
This is important to keep in mind, because the S76 in question had flown into fog. Zero visual references. It had also entered a climb and lost airspeed, enough to where it appears to have transitioned from forward flight (where it has the natural stability of an aircraft) into hover with zero stability.
The instruments in most commercial helicopters are the same as in an aircraft, and thus only useful if the helicopter has sufficient forward velocity to be in forward flight mode, i.e. > 30 kts for an aircraft the size and weight of the S76. One cannot hover a helicopter with aircraft instruments, on instruments alone. They are not precise enough. Some military aircraft have additional instruments that provide the precise attitude and acceleration information to allow hover with zero visual references, typically SAR or special ops birds.
What I didn't know until today: a helicopter with aircraft style instruments cannot be successfully hovered in IMC conditions on those instruments alone. Absent visual references or helicopter specific instruments, it will become increasingly unstable and crash. And do so fairly quickly, like within 30 seconds.

I have removed the big section in the middle where you speculate on an accident still under investigation. Please don't do that. The rest of this does not answer the question asked.

authors: Vladimir F Героям слава


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Why do some airlines not use containers for luggage?
Many planes allow the use of ULD containers to store hold luggage: such luggage is placed into the containers, then the containers are stored in the hold of the plane.
There are many advantages to this, as explained here: Why is luggage required to be loaded in containers?
However, some airlines do not use ULD containers even when that is possible. I have in mind Easyjet on their A320-family planes, which don't use containers, at least on some flights, even though the planes definitely can handle them.
What could be the possible reasons for this? Given the high emphasis on saving every single penny they can, I suppose they have tried both approaches and come to the conclusion that not using ULDs is actually somehow cheaper than using them, which seems counter-intuitive:

it should take longer to load and unload luggage one by one than just move containers in
more manipulations increases the risk of damage to the luggage

Possible reasons I can guess at (but not verify) are:

loading the containers is actually not so much faster, with containers possibly getting stuck or some similar problem?
loading containers requires more equipment, and turns out to be more expensive for this reason
it depends on the amount of luggage being carried?

Does anyone have any insights on this?

Not sure, but it might have to do with the infrastructure. Companies like Easyjet often operate between smaller airports that lack the necessary facilities for loading and unloading ULD's. In such cases it could be less efficient to use them.

@RobVermeulen, I've seen this on LGW-TLS flights, and I expect both airports to be equipped for ULD handling. Of course, they may pick a standard procedure for all flights if some airports are not capable of handling ULDs.

@mins, I already linked to that question, and all the answers are in favour of using ULDs, while I'm trying to understand why some airlines don't use them.

The ULD add extra weight in some way, while using luggage nets should not add any significant load. Furthermore maybe loading and unloading time of smaller aircrafts isn't such a matter at all. A B747 or A380 carry 410 to 550 passengers at once, while a A320 carries only 100 to 200 passengers.

A good example is mabye the airport FMM in southern germany. With only a handful of flight operations during a day. I don't think they can unload ULDs there.  https://de.wikipedia.org/wiki/Flughafen_Memmingen#/media/File:FlughafenMemmingenVorfeldRyanair.jpg

Containers make sense on long-haul flights, but don't make much sense for short hop (which are usually connecting flights), where the baggage needs to be removed from the containers and resorted at each location. At that point its easier just to skip the container.

@RonBeyer, I'm not sure that makes much sense. One of the goals of using ULDs is that it means: load luggage one by one into ULDs (time consuming), load ULD into plane (supposedly quick), and vice-versa at the other end, compared to put luggage onto trolleys one by one (time consuming) then load luggage from trolleys onto plane one by one (time consuming). This does not change whether the flight is short or long. Also the example is a low-cost carrier, which usually does not manage connecting flights.

One thing to note with Easyjet (and other low-cost carriers) is that they discourage hold usage entirely: a large proportion of low-cost carrier passengers will take hand luggage only, as there is often a surcharge for a hold bag. As such, they carry smaller numbers of hold baggage items than other carriers

There are some siuations where it is better to load the luggage individually than in ULDs.

The airports served by the flight should have the necessary infrastructure to handle them. As an example, a number of carriers have outsourced their ULD logistics, which are available only in a few (usually long haul) destinations.
The ULDs are fast and economical (compared to putting luggage directly) only if there is some minimum amount of luggage. For short routes, the use of ULDs may not offer any significant benefits compared to direct loading due to the amount of luggage. Plus, the ULDs themselves add (some) weight, which should be small compared to the luggage carried.
Another thing is that for connecting flights (i.e. only a few passengers are transferred), it is better to have the luggage in the hold rather than in ULDs as the transfer time required would be lower (U don't have to open the ULD to get their baggage).

I suspect the main reason is economics. This presentation talks about the pricing of ULDs and describes two rates:

A base rate for use of each ULD.
An another rate for weights above the allowable weight.

So, if the airline is 'renting' the ULD (as I suspect most of them are), it doesn't make sense for them from an economic point of view to 'rent' an ULD unless the amount of luggage is above some threshold value, which is satisfied only in long routes or the number of passengers is more. So, low cost carriers may totally skip using ULDs and charge their customers for extra luggage to discourage them from carrying it (they can use the space and weight for cargo instead, though I'm not sure about it).

In addition to the reasons given by @aeroalias, cargo containers also waste space.
From the A320/321 Operator's Manual:

The A320 can hold up to 7 containers. The A319/320/321 do not use the standard LD3 containers. They use the slightly smaller AKH container.

Each of these has a volume of 3.7 m3, so seven of them totals 25.9 m3. When bulk loaded the A320 has 37.28 m3 of cargo space. That's almost 44% more room.

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Why locate engines far from the fuselage in some aircraft, such as the SR-71?
There are some plane designs where the engines are placed quite far away from the fuselage, one example is the SR-71.

This amplifies the effects of any asymmetry of the engine thrusts, the worst case being zero thrust on one side. What are the reasons that justify or force those designs?

What makes you think the engines on the SR-71 are far from the fuselage? They are similar to the 737 which is about the same size.

If you want to drop paratroopers from the forward doors, it helps when the propellers are far enough away to not shred them once they leave the plane.

@PeterKämpf You just gave me a mental image of paratroopers being dropped from an SR-71. I strongly suggest you work together with Michael Bay.

One other reason would be cabin sound level - the further the engines, the less acoustic dampening necessary

@sanchises: And I strongly suggest you work on your mental images if you connect propellers with the SR-71 ;-)

@PeterKämpf Classified alien technology. Wait, I have proof! :)

The asymmetric yaw from a dead engine can be extremely large, take for instance a 777...yet that plane handles fine OEI simply because it has plenty of rudder authority to let the pilot deal with that yaw.

From Wikipedia:

Placing engines on the wing provides beneficial wing bending relief in
  flight. The further the engines are away from the fuselage the greater
  the wing bending relief so engines buried in the wing root provide
  little relief. Almost all modern large jet airplanes use engines in
  pods located a significant distance from the wing root for substantial
  wing bending relief.

One could imagine that, in flight, the wings are holding up the aircraft, but this causes a massive bending moment at the wing roots (just visualize  an airplane suspended by ropes from the wings). As such, is favourable to have the airplane body be as light as possible, and put as much mass (e.g., fuel) as possible on the wings, where the lift forces coincide (or at least, are closer to) the weight of the engine, reducing bending moment on the wing roots. This allows for smaller (and thus more aerodynamically efficient) wing structures.
As noted in the comments, placing the engines further away of course increases downwards bending moment when the plane has landed. However, this will be 1g by definition (at touchdown, the wings still provide lift (generally)), whereas during the flight, an airplane is designed to take up to 2.5g (and, I suppose not completely coincidentally, -1g).

On the other hand you have more stress on the wings while standing or touching down. There must be an optimum distance considering this, but I guess that is not too far from the fuselage as in many designs the engines are attached directly to the fuselage.

On-body engines are generally due to other design considerations, such as ground clearance (which is why small jets have tail-mounted engines so that you can have a door-sized staircase) But as far as your 'optimum distance' concerned: that distance would be almost exactly the distance that is used on modern jets - it is inconceivable that airliner manufacturers have not optimized this. Note that engine placement is also dependent on aerodynamic effects (e.g., wing flutter), not just weight.

I totally agree with your considerations on (boring) airliners. I was just wondering about less common designs where the engines looks quite far away :)

@DarioP, I think for the SR-71 case Victor Juliet's answer is partly correct - aerodynamics around the turbine inlet.  Not a safe design if one stopped though (some Blackbirds were lost this way).

@DarioP I tried to keep the answer general, but in the case of the SR71, I can make two guesses. First, I think the engines need to be ahead of the shock wave made by the nose, while still keeping the aircraft sleek (i.e. elongated), see VJ's answer. Secondly, I think the SR71 is capable of pulling more than 2.5g, and look at the massive engines compared to the flimsy wings.

There are a couple of reasons which drive the need to place the engines at optimum distance from the fuselage. In case of Blackbird SR-71, the engines have to be placed at a distance such that they can ingest optimum amount of air for proper combustion. Placing the engines too close to the fuselage would hinder the air flow, and hence the engines might not work properly. The CONS of having such a placement, as mentioned in the question can be done away by having proper damping, and design techniques to reduce the effect of any kind of asymmetry.

The SR-71 was a pretty extreme example - I've read it was very nasty if one engine stopped.  Maybe damping, etc. would be OK for less extreme aircraft though.

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Why is radar required on some FAA instrument approaches?
The ILS / LOC approach at KCXY states radar is required. I understand I can't fly the approach if not in radar contact. However, if flying the LOC approach I think I can identify both final and HORVI (for lower minimums) using HAR VOR. There's no reason I'd need the controller to call out a fix. If that is true, why is radar required? I think it would be more of a safety concern because of terrain and an adjacent airport (HAR Int)?

An observation.  There is a RADAR note under TIVNE.  This clues the pilot into the fact ATC has that waypoint on their screen.  If you ask them to tell you when you pass it, they are required to.

Radar is most likely required in this case because there is no defined Initial Approach Fix (IAF). This means that ATC needs to give vectors to final or otherwise provide a specific clearance to establish an IFR aircraft on the approach.
There may be other considerations, but that is the main one that I see here.

Good call. Totally missed no IAF on approach plate.

Well, the question would then become: Why is there no IAF?

@Lnafziger - my assumption would be because they want to know and have contact with all aircraft in that area. This could be because of the terrain to the SE of runway or the airport that is 3NM to the E (marked on plate as Harrisburg International) that has an ILS over CXY.

As mentioned above, this approach has no IAF.
That's because the the VOR is at 1,301 MSL (note the height of the obstacle next to the VOR is 1,410), and the segment from TIVNE to HORVI descends from 2,800 to 1,340 MSL, and will dip below the usable volume of a Low Altitude VOR. According to AIM 1-1-8, a low altitude VOR with a standard service volume cannot be received inside 10 miles and below 1,000 feet above the NAVAID.

Excellent detective work, I think that this best answers the reason of why.

As I was trying to figure out why the VOR was unusable in the segment previously described, I realized that my entire answer was wrong, and fixed it.

I'm not sure that is correct rbp. Via your link section c "Standard Service Volume limitations do not apply to published IFR routes or procedures."

@Clarke76 even the FAA can't reverse the physical limitations of transmitting below the VORs line of sight.

@rbp Glad we are having this discussion. Could even be it's own question on here. That being said the FAA wouldn't have a rule overriding SSV if it wasn't used. They also wouldn't clearly mark HORVI with HAR 142 radial. It wouldn't be on the plate if you couldn't. They must have determined there was adequate signal, 39ft above VOR, to id fix. I'd go so far as to say there are many approaches that use VOR to id fix that are not within SSV numbers. (ILS or VOR RWY 8 @ KLNS for example).

From the FAA's hand book on IFR flying (page 4-17). It seems that it is the minimum equipment required to fly the approach.

In some cases, other types of navigation systems, including radar, are
  required to execute other portions of the approach or to navigate to
  the IAF (for example, an NDB procedure turn to an ILS, or an NDB in
  the missed approach, or radar required to join the procedure or
  identify a fix). When ATC radar or other equipment is required for
  procedure entry from the en route environment, a note is charted in
  the plan view of the approach procedure chart (for example, RADAR
  REQUIRED or AUTOMATIC DIRECTION FINDER (ADF) REQUIRED). When radar or
  other equipment is required on portions of the procedure outside the
  final approach segment, including the missed approach, a note is
  charted in the notes box of the pilot briefing portion of the approach
  chart (for example, RADAR REQUIRED or DISTANCE MEASURING EQUIPMENT
  (DME) REQUIRED). Notes are not charted when VOR is required outside
  the final approach segment. Pilots should ensure that the aircraft is
  equipped with the required NAVAIDs to execute the approach, including
  the missed approach. Refer to the AIM paragraph 5-4-5 for additional
  options with regards to equipment requirements for IAPs.

Later on they talk about Radar Approaches (Page 4-71)

The two types of radar approaches available to pilots when operating
  in the NAS are precision approach radar (PAR) and airport surveillance
  radar (ASR). Radar approaches may be given to any aircraft at the
  pilot’s request. ATC may also offer radar approach options to aircraft
  in distress regardless of the weather conditions or as necessary to
  expedite traffic. Despite the control exercised by ATC in a radar
  approach environment, it remains the pilot’s responsibility to ensure
  the approach and landing minimums listed for the approach are
  appropriate for the existing weather conditions considering personal
  approach criteria certification and company OpSpecs.

They can also be useful in helping disabled aircraft in (page 4-74).

Perhaps the greatest benefit of either type of radar approach is the
  ability to use radar to execute a no gyro approach. Assuming standard
  rate turns, ATC can indicate when to begin and end turns. If
  available, pilots should make use of this approach when the heading
  indicator has failed and partial panel instrument flying is required.

You appear to be explaining what "Radar Required" means, not explaining why it might appear on an approach.

Good citation regarding procedure entry from the enroute environment. However, I really don't think the PAR and ASR approaches have anything to do with this question, certainly not the PAR.

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/


text:
Why aren't departures separated by minimum distance rather than minimum time?
Normally, two departures are required to be separated by a minimum amount of time depending on the weight class of the aircraft.  However, I was wondering if someone could explain why departure-departure separations are not assigned by a minimum distance instead of a minimum time.  It seems that using a minimum time is potentially more inefficient, especially if the time to reach a safe distance for wake effect purposes is considerably less than the minimum.
As a related question, are there any research projects or articles that someone can point to that talk more about this subject?
Thanks!

I understand that the reason for the time rather than distance separation is that passive wake turbulence avoidance by dissipation is primarily a time dependent matter.

For the FAA(all my answers can be looked up, and expanded upon in FAA Order 7110.65W, Chapter 3 mainly, but I'll try to keep it short), most departures from the same runway use distance for separation. The standard is the lead aircraft is clear of the runway, but most of the time can be shorter under most circumstances. The only times it becomes time based, if it's wake turbulence related, or various conditions of non-radar separation needing to be applied.
In general for runways, aircraft are broken into 3 categories for separation(excluding weight class, which add wake turbulence time if necessary). Category I aircraft are light single engine propeller driven aircraft weighing less than 12,500 lbs (and helicopters). Category II aircraft are light twin engine propeller driven aircraft weighing less than 12,500 lbs. Category III is everything else.
I'll mainly discuss same runway departures, since there's many considerations and it gets complex, and the FAA Order 7110.65 does a fairly good job of depicting and showing this separation standard. For the same runway when both aircraft aircraft departing are category I, the min distance between them is 3,000 feet and airborne. This means that the first aircraft has to be 3,000 feet down the runway and airborne, before the next aircraft starts it's takeoff roll. If the trailing aircraft is a Category II and the first aircraft is a Category I or II,  the distance increases to a separation of 4,500 feet between aircraft. If either aircraft is a Category III aircraft, it's 6,000 feet and airborne required.
Now if both aircraft are IFR, there needs to be IFR separation insured after the runway separation. In most environments the minimum radar separation is 3 nautical miles between aircraft. Now, between jets, the runway separation of 6,000 feet and airborne, will often be right at 3nm between aircraft(due to the speed differential mainly). Other options include turning one aircraft off the path of the other by at least 15 degrees and have them diverge apart.
Wake turbulence is one of the few areas that time is applied. If a heavy(300,000 lbs or greater) is departing in front of anything else, the next aircraft can't start their departure roll for 2 minutes when both are departing from the same intersection on the runway, and 3 minutes if departing the runway from different intersections. And a Super(A380/AN225) I believe are 3 minutes, when departing the same intersection, but I've never worked them in the tower. There are a few other times when time is used, intersections, opposite direction.
For arrivals, all except non-radar separation is distance based.


text:
What is this delta-winged aircraft?
I spotted this recently. The distance and poor camera makes it really hard to determine what it is. Any help appreciated. Thanks.

Could it be a kite?

It could be some kind of top secret aircraft.  Did you take this picture and if you did than where.

Is it a screenshot of a Star Wars film?

Is it a paper plane?

It's a UFO

Took a fair bit of trial and error on the very vague registration on the port wing :)
F-PDHV (D-Plane 1)

The Verhees D-Plane 1 is a Belgian homebuilt flying wing,
  designed by Verhees Engineering and supplied as plans for amateur
  construction.

Picture from below:

Source

David Osborne/ Aeroprints Source

great sleuth work!

Wow, that's very interesting. I saw the picture and assumed it was fighter or bomber sized!

@Dan This is often the problem with photos that have no frame of reference: You can't tell if it's something really big flying very high, or something fairly small flying very low.

@Dan And I thought it was a UFO!  (Which is was... until MikeFoxtrot identified it...)

@Dan, I was assuming the other end of the scale: somebody's toy plane or drone.

and i almost thought its a top secret aircraft and the OP of question will be in a heap of trouble soon :D

@RicoRicochet If a regular civvie can snap a photo of it in plain view, it's pretty certain that it's not a top secret aircraft. ;)

@J... in fact it wasn't an aircraft at all.  That was just swamp gas you saw.

There's a great real-life example of mistaken assumption by a very experienced witness, due to lack of frame of reference, in the second answer here.


text:
What are some examples of warning areas and restricted areas?
Can you please give me some examples of warning and restricted areas?
How can an aircraft enter these areas? What are the procedures you have to follow before entering them?

Yes Pls. Its okay if it is other country.

Start reading here http://www.faraim.org/aim/aim-4-03-14-143.html

I'll provide information for the US airspace. However many countries, ICAO members, will have similar concepts and procedures.
Short answer
You are not required to follow any procedure for entering a warning area or an inactive restricted airspace. In the latter case, you need to ensure the area is actually inactive. A safe practice is to contact the controlling agency of the area prior to enter any warning or restricted one.
The longer answer below develops a bit around special use airspaces which encompass warning and restricted airspaces among other. You'll also find two examples of these areas.

Context
The airspace over the US is open to navigation, a part of this airspace benefit from services provided by air traffic control (ATC), e.g. safe separation and traffic information.
This controlled airspace includes airport areas and airways, and other areas of interest. It is is ruled by air regulations relative to airspace classes. In the US these regulations are found in document 94 CFR 91.

source)
Some limited areas, that may be permanent or temporary, can be prohibited or restricted. Together they constitute the special use airspaces (SUA)

Special use airspaces
SUA (Wikipedia):

Special use airspace includes: restricted airspace, prohibited
  airspace, military operations areas (MOA), warning areas, alert areas,
  temporary flight restriction (TFR), national security areas, and
  controlled firing areas.

The SUA are repertoried by the FAA: SUA Web site.
You may search by name or scan the map. R denotes a restricted airspace, and W a warning area.

More on SUA: Course Notes, Dennis Seals, FAA
Your question is limited to warning areas and restricted airspace.

Restricted airspace
Wikipedia:

Restricted areas denote the existence of unusual, often invisible,
  hazards to aircraft such as artillery firing, aerial gunnery, or
  guided missiles. Penetration of restricted areas without authorization
  from the using or controlling agency may be extremely hazardous to the
  aircraft and its occupants.

See also chapter 3−4−3. Restricted Areas of the FAA-AIM.
Entry: Forbidden when active, subject to ATC clearance on a case by case basis.
For areas that have a scheduled activity, times of activation are published in the AIP. When inactive, a R-area is just a like any other location of the surrounding airspace class. When active it is just forbidden to enter it (albeit FAA may deliver specific clearances if solicited).
For VFR flights, contacting ATC prior to entering an inactive restricted area is not mandatory per regulations, but is a good practice regarding safety. IFR flights under monitoring of the ATC, will not be authorized to enter an active restricted area.
Wikipedia:

Restricted airspace zones may not be active at all times; in such
  cases there are typically schedules of local dates and times available
  to aviators specifying when the zone is active, and at other times,
  the airspace is subject to normal VFR/IFR operation for the applicable
  airspace class.

Activity time may happen unscheduled for some R-areas. A schedule cannot be published, in this case a NOTAM is used to inform airspace users.
Wikipedia:

A few zones are activated by NOTAM; an example is R-2503D over Camp
  Pendleton in southern California, between San Diego and Los Angeles.
  This particular zone, beginning at 2000ft above sea level over most of
  southern Camp Pendleton, can only be active for a certain number of
  days per year, thus allowing small planes to fly a direct route over
  land between the two metro areas instead of being diverted offshore or
  into mountainous terrain further inland.

Example: Restricted areas R-2501 around the Twentynine Palms Marine Corps Air Ground Combat Center:

GlobalSecurity.org:

The Twentynine Palms Complex is located in the Southern California
  desert approximately 115 NMI northeast of Los Angeles, CA. The
  Twentynine Palms Complex provides a vast land and restricted airspace
  (R-2501) area for live ordnance employment and combined arms training
  of infantry units, armored vehicles, artillery, and air support.
The complex includes the following instrumented area: R-2501E
The following are training, and Military Operating Areas (MOAs)
  associated with the Twentynine Palms complex:

R-2501N
R-2501W
R-2501S
Bristol MOA
Sundance MOA
Range Training Areas (RTA)

Description of R-2501N from Order JO 7400.8S:
R-2501N Bullion Mountains North, CA
Boundaries.
Beginning at lat. 34°30'00"N., lon g. 116°26'23"W.;
to lat. 34°36'00"N., long. 116°28'03"W.;
to lat. 34°40'30"N., long. 116°29'43"W.;
to lat. 34°43'00"N., [...]
Designated altitudes. Unlimited.
Time of designation. Continuous.
Controlling agency. FAA, Los Angeles ARTCC.
Using agency. Commanding General, Marine Corps Base, Twentynine Palms, CA.

This area around a large military complex is active (restricted) 24x7, at any altitude. A pilot who wants to enter this area for some reason needs to contact either the controlling agency (Los Angeles ARTCC) or the using agency (Marine Corps Base).

Warning area
Chapter 3−4−4. Warning Areas of the FAA-AIM.

A warning area is airspace of defined dimensions, extending  from
  three nautical miles outward from the coast of the U.S., that contains
  activity that may be hazardous to nonparticipating aircraft. The
  purpose of such warning areas is to warn nonparticipating pilots  of
  the potential danger. A warning area may be located  over domestic or
  international waters or both.

Entry without specific clearance, but can be hazardous. Contacting ATC prior to entering a warning area is not mandatory per regulations, but is a good practice regarding safety.
Example: Pacific Missile Range Facility (PMRF) NS Barking Sands.

GlobalSecurity.org:

The area surrounding Kauai is divided into warning areas with W-186
  and W-188 controlled by PMRF. The Fleet Area Control and Surveillance
  Facility (FACSFAC) controls W-187, 189, and 190. Space, air, and
  surface tracking are accomplished from PMRF precision-tracking radar
  sites at elevations of 75 ft., 1700 ft., and 3800 ft. These are
  supported by radar systems operated by agencies external to PMRF.

Description of W-188 from Order JO 7400.8S:
W-188 Hawaii, HI
Boundaries. Beginning at lat. 21°58'19"N., long. 159°48'45"W.;
to lat. 21°58'27"N., long. 159°59'50"W.; [...]
Altitudes. Surface to unlimited.
Times of use. Continuous.
Controlling agency. FAA, FAA, Honolulu Control Facility.
Using agency. Commander, Pacific Missile Range Facility, HI.

More:

Related question: Can a pilot fly through a restricted area while flying on a Victor Airway?

They serve the same basic purpose, most generally military training areas.
The distinction is that restricted areas are all over land, most have a ceiling at 18000 feet and are generally isolated so that routing around is usually only a small diversion, they also comply only with US/FAA standards. Restricted differ from an MOA in the frequency of use and level of activity, as such you are not required to contact the controlling agency before entering an MOA (even if active)[but you should for your own safety], you must contact the controller before entering a restricted area.
Warning areas are over coastal waters and are operated with consideration for ICAO standards, warning areas are individually larger than restricted areas and are mostly in a continuous series so routing around them can be more trouble.
The specific times, altitudes, and contact frequencies are listed on the legend portion of low altitude charts. They may be active by a regular published schedule listed on the chart legend, or by notam.


text:
Can a captain fly in the right seat as SIC under part 129?
Re a Part 129 air carrier that is short of co-pilots:  Can a pilot who usually flies in the left seat (B737 aircraft: non-US registry) as the Pilot-in-Command fly in the right seat as the SIC (co-pilot)?

From the regulations:

Each person acting as a flightcrew member must hold a certificate or
  license that shows the person's ability to perform duties in
  connection with the operation of the aircraft. The certificate or
  license must have been issued or rendered valid by:
(a) The State in which the aircraft is registered; or
(b) The State of the Operator, provided that the State of the Operator
  and the State of Registry have entered into an agreement under Article
  83bis of the Convention on International Civil Aviation that covers
  the aircraft.

Since a captain can always fulfill the duties of a first officer, then yes, someone who normally occupies the left seat can occupy the right seat.

Thank you, Simon.  As a follow up....if the Captain has never flown in the right seat, your position is that he may do so without any right seat certification?  How about right seat landings and any other requirement, such as operation of avionics equipment that is only accessible from the right seat?

Without disagreeing with @Simon's answer, let me add that the actual OpSpecs and procedures at the particular carrier will also play a role. If a captain can routinely occupy the right seat, then he probably gets at least a minimal amount of training in it annually -- perhaps one takeoff, approach, and landing, as well as any duties specific to that seat. I've seen one carrier that used to allow 2-captain crews that decided to discontinue the practice, so now only Check Airmen do the additional right seat events on a Captain sim ride, and thus only they can fly right seat - not line captains.

@LeeBauer - That's not how it works. You don't get to be Captain of an airliner without spending 1000 hours as First Officer. In addition, the Captain and FO work together on most tasks of the airliner, so the Captain knows what the FO should be doing and v.v. So if a Captain ever has to take the right-hand seat for a flight, he'll know what's up.

That's the key.  You can't sit on the left, unless you've done a lot of time, and by definition are certified, in the right seat.

@KeithS  He's a captain because he did time as an FO, but not necessarily in the same aircraft.  737 FO to 767 FO to MD80 CA is entirely possible, and the knowledge he has for his left seat duties doesn't necessarily translate into proficiency in all the hardware that only the right seat has access to.  Not a huge number of things, but maybe the ACARS in an MD-80, or specific Cat II or Cat III duties are different, and knowing what the other guy is supposed to be doing isn't always the same as being ABLE to accomplish it in a timely manner yourself.

@Simon  That's entirely incorrect.  There is zero requirement in the FARs or in many company policies & OpSpecs to have anything close to "a lot of time" in the right seat in order to be qualified in the left seat.  Now, in many cases the pilot does have right seat time in type, and in many cases the airline keeps him qualified in that seat, but the suggestion that it is a "requirement" to have that experience in the type being flown simply doesn't match the facts at many US carriers.

@RalphJ I didn't say on type.  Are you aware of any operator that does not require that both crew can fulfil the duties of the other if required?  I'd be keen to know so that I can add them to my "no fly" list ;)

@RalphJ - The requirements changed in 2013: http://aviationblog.dallasnews.com/2013/07/15604.html/. Both Captain and FO need an ATPL (1,500 hours TFT) with type rating, and FOs need 1000 hours in the right-hand seat as FO before they can fly as Captain. I'm sure there's some sort of grandfathering for existing flight crews but someone working their way up the ladder today has a pretty steep climb to Captain.

@KeithS   He has been in the right seat of SOMETHING, but not necessarily of the aircraft he is flying as Captain. Thus the 737 FO to 767 FO to MD-80 Captain example.  The pilot can be an MD-80 captain with ZERO right seat time in the MD-80, which is why his airline may choose to NOT consider him qualified to sit right seat, only left seat. The rule quoted doesn't change that dynamic.

@Simon -- yes, very very familiar with a carrier that no longer allows 2-captain crews (check airmen excepted) for a particular fleet.  A change they made as the right seat role became more specialized.  And no, I'm not telling you who. Let's just say you've certainly heard of them.

I don't believe that this answers the question.  The regulation that you quote (by the way, it would be helpful to state which regulation it is when you quote it) is but one of many many regulations that must be complied with by the pilot of an air carrier.  Simply holding the "certificate or license" does not qualify them to act as an air carrier crew member, which this implies.  There are many training requirements, some of which could apply here...

authors: كَـــلِــمُــنْ KLMN


text:
Is it feasible to have passenger jet aircraft seats turn into bouncing cocoons on impact?
Why not engineer a design where in an emergency, the seats turn into a cocoon capable of bouncing on impact and encase the passenger in such a way that impact with the ground will not harm him. of course the seat would have to be built of some new material maybe graphene that is 100's of times stronger than steel and would have to have points of contact with the passenger's body to prevent jarring the body's organs etc. Now cost maybe prohibitive but I think engineers can devise such a seat mechanism that may be light enough but yet strong enough to do the job. is it possible?

Why not just invent a teleporter so people don't have to get into an aircraft in the first place?

They haven't even done that for cars. With modern airbags etc car crashes are survivable: but not necessarily at speeds of 70 mph or more.

The Science of Survivability

@ChrisW That just makes me wonder why there's no airbags on planes :p

@DannyBeckett Probably because the types of accidents for which they're likely to be useful are extremely rare in aircraft. In the case of a passenger cabin, they'd also be more likely to impede an evacuation than actually do anything productive.

If we assume an initial speed of 150MPH (modest takeoff or landing speed of a modern airliner) and a cocoon providing a 1m (~3ft) cushion, the subject could experience deceleration forces on the order of 200g. Reduce the cushion and/or increase the initial speed to what an airliner really flies at and the g forces go up from there. Consider that 100g is likely not survivable. Possible, perhaps... likely, no. To ensure that g forces are survivable, a cocoon would likely have to be several hundred feet across.

In a word, cost.
Weight equals cost.

How much is this seat going to weigh?

How many paying passengers will not be able to fly because of the added weight of the seats (and the associated cocoon-deploying equipment)?

How much more fuel will the airplane burn to carry all this extra weight?

Design equals cost

This super-strong material doesn't exist yet. How much will it cost to invent? Who will pay to develop this material?

How much does the material itself cost? If it's so wonderful why don't we make the whole plane out of it, as every stand-up comic suggests?

How much will the research to perfect and certify the deployment technology cost?

Passengers panic when the oxygen masks drop, we can't have the cocoon seats deploying unless there's an actual emergency.

What if there is an actual emergency and the seats don't work?

Deployment equals cost

How do passengers evacuate the aircraft if they're trapped in cocoons? Do emergency crews have to cut into the structure of the aircraft to get them out?  Now the airplane is destroyed. New airplanes cost money.

What if the aircraft ditches in the Hudson river? Do all the passengers drown? Maybe we should make all the seats eject out of the aircraft. Guess what?  Destroyed aircraft still cost lots of money and now you've got the extra weight of the ejection devices.


text:
Why is paragliding not an old invention?
Paraglider looks like a rather simple device for me. It does not have a complex engine that needs lots of technology to be built. It also does not need a powered aircraft just to be launched. Looking from the side I have an impression that it can cover serious distances along the ridge under right wind. While it is not convenient to have the airways bound to mountain ridges, it could still have had uses like patrolling borders or similar.
I am surprised, why the master that could build a bow, or possibly sails for the ship could not build a paraglider many years ago. What are the reasons why the paraglider is relatively recent invention? Is there something complex about is I am not aware of?

Just a thought, since you don't know the subject. Do you actually mean a paraglider, or do you mean a hang-glider? The two are radically different aircraft!

Hang glider has aluminium alloy or composite frame so I understand it requires advanced technology. It is not just silk and threads. If to make from wood, it would be a glider but probably too heavy for launching on foot.

@h22 Modern hang gliders were built with bamboo or spruce frames and plastic tarp material for  about a decade before they started appearing with aircraft tubing frames and ripstop nylon sails.

@h22 You have that exactly reversed.  Otto Lilienthal built and flew hang-gliders made of cloth and light wooden canes, back in the 1890s.  His problem was the aerodynamics and weight-shift control authority, but there was absolutely no problem making it fly.  In the 1960s, hang-gliders based on the Rogallo design again just used bamboo and tarps, with aluminium where available.  Hang-gliders are relatively simple in concept and need relatively slack tolerances, at least if you're not building a serious racing machine.

@h22 ... Conversely a paraglider is right up against the limits of aerodynamics, and the ram-air structure makes it very complex.  It also needs very, very light fabric to allow the ram-air structure to inflate, compared to a regular chute which can get away with heavier fabric because you're just on a one-way trip down.  Before the discovery of artificial fibres such as nylon, even the finest silk might not be light enough.

Paragliders as we know them depend on parawings with relatively high glide ratios and useful levels of steering.  Neither of these qualities were well represented in the round parachutes (even the so-called glide 'chutes) in common use before modern flat canopy glide parachutes came along (in the 1970s?).  Parawings as we know them were an outgrowth of these highly maneuverable parachutes, modified to have lower loading (for better sink rate) and optimized for horizontal flight, without the need to open in a near-vertical fall at terminal velocity.
If the knowledge had existed, parawings as we know them could have been built as early as the 1860s (when plain, round, uncontrollable silk parachutes came into use for stunt jumps from balloons).  The missing ingredient, as it were, was simply the knowledge that such a thing was useful and possible.

Furthermore, the materials required to build light yet airtight paraglider wings are quite recent inventions. (Unlike a controllable parachute, paragliders have to be airtight to some degree in order to form a real wing and therefore result in a glide ratio which allows effective climb when flying in updrafts.) Kevlar used for the lines which is both strong and light wasn't available before 1971.

Silk, as used for balloon envelopes, could have filled both roles in the 1860s.  Lines might need to be a bit larger than Kevlar for the same load, and you'd want to avoid getting the thing wet (silk stretches when wet), but balloon silk, sewn with silk thread, and silk cordage could do the job.

I disagree with the useful and possible part. Humans saw birds doing it so knew it's possible, there has been mythology from many cultures where humans have flown, the story of Icarus comes to mind. It's not been from want of motivation.

"Possible" -- possible for birds and possible for humans are very different conditions.  The principles underlying heavier than air flight were still very mysterious in the 1860s.  They weren't even fully understood in the early 20th century when everyone with a lot of money and a sense of adventure was building widowmaker airplanes.

I was jumping at the start of the Ram Air revolution in the mid-late 70s, when a 7 cell canopy called the Strato Cloud was all the rage.  I had a Paracommander, the heavily slotted round canopy that was most popular up to that time, and which could glide about 1/1.  The Strato Cloud had an LD of a rogallo hang glider, about 4 or 5/1.  I couldn't afford a Strat.  An up to date rig with an SC, tandem container with single-point cutaway system, and tri-conical reserve was about 3-4k in 1977 dollars.  You could buy a small car for that.

@JohnK In some ways, a parawing is simpler -- I recall reading that a lot of thought and effort went into making the flat canopies open slowly enough not to injure the jumper.  Parawings are laid out on the ground and then the leading edge pulled up to get them to fill and lift off, very much like a power kite.

And yes, a new VW Super Beetle was just over $3k in 1977.  A Chevette or Pinto was similarly priced.

I think the parawing is safer than the Pinto @ZeissIkon!

@GdD There's also little justification for "useful". Paragliders are the worst form of glider available by every measure of flying performance. If you have enough lift to maybe get a paraglider in the air in a hundred feet of lift band, a sailplane can stay aloft all day and explore a few miles away from the ridge before coming back for a top-up of height. They're fun, sure, but they aren't "useful" for any meaningful work. Skateboards are fun too, but they aren't a major transportation type either.

@ZeissIkon That doesn't make them simpler, it just makes them different. The challenge with a paraglider is stopping it collapsing in turbulence. They need millimetre-accurate Kevlar lines, and some really complex aerodynamics, and that's why so many paraglider pilots died in the 80s and 90s.

@ZeissIkon there were two deployment systems in common use back then, the square panel slider, and a system that did something similar but connected to the pilot chute bridle itself to do the same thing. "Style" jumpers, who did a kind a kind of acrobatic freefall routine and as a result opened at well above normal terminal, preferred the pilot chute bridle system as it worked better to slow opening at high speed.  A style jumper using a slider on his rig had to suffer through brutal opening shock.  Of course the slider seems to have won out for its simplicity.

@GdD Oh, and just by the way, you had to be pushing the Pinto pretty hard before the tyres failed. A paraglider OTOH can and does collapse simply through the wind blowing wrong. Modern gliders are much less likely to, but it took 20-30 years to get to that point. I started flying hang-gliders in the 90s. I knew a number of paragliders who were left with severe injuries when their canopy collapsed too low to reopen. Air-bag harnesses have been a major step forward in safety, of course, but the fact remains that a paraglider doesn't hold its shape.

I was joking about the Pinto @Graham.

Ancient manned flights used either "birdman" wings or kites. Nobody even understood the problem of control, never mind solved it, and both were often fatal, birdman wings especially.
That did not change until in the first part of the 19th century Sir George Cayley defined control as a specific problem to be solved, and that would take almost a hundred years more. Meanwhile, early attempts at parachutes were uncontrollable and swung wildly, sometimes fatally.
But even kites still had rigid sticks to hold their shape, the idea of a wholly non-rigid aircraft, shaped by the airflow, seems not to have appeared until kiting enthusiasts Francis and Gertrude Rogallo came up with it shortly after WWII. The control system was key to making it work. I am not sure who first invented the parawing, but Francis Rogallo subsequently adopted the technology in his day job, designing lightweight spacecraft recovery systems, and went on to popularise it along with other kite-derived flexible wing forms. For a time both the parawing and the biconical delta (the latter patented by others back in 1909 and 1910, and as a kite just a year before the Rogallos' unrelated patent) were known as "Rogallo wings".

It was less than one hundred years from Cayley's carriageman quitting his job (because Cayley's flying steam coach nearly killed him) to the First World War, by which time the control issue was undeniably solved.

Woops, Cayley's was a glider, but Cayley was born only just over a century before the Wrights demonstrated controlled powered flight.

Well spotted. I have corrected my timing. I suppose that "almost" is a matter of opinion.

Thanks! I was surprised to find out that basically history of early aviation is 'waiting until everyone realizes the real problem to solve is control' - that's what Wright brothers did (plus finding a light enough engine).

An airfoil made only of textiles consists of many air cells which are open at their leading edge.
"it's easy to make an airfoil from only strings and textiles and make a parachute", is perhaps not so easy! it's non-obvious idea which is complex in design.
The first guy to fly a parachute thought of it when he wanted to escape his jail cell. after he was freed he pursued the idea until he had the first jump from a hot air baloon in 1797. it took 153 years until the parachute was shaped like an airfoil, because airfoil technology was developed in the 1900's.
https://www.history.com/this-day-in-history/the-first-parachutist

This is a great answer, it may even be that the right one!

Material:

You cannot build today's paragliders without modern materials The ropes need to withstand several kN of force while keeping diameter to a minimum to reduce drag. The glider must be almost impenetrable for pressurized air, which is impossible with cotton.
If you build a paraglider with classic materials, it will be too heavy to fly and way too prone to collapse mid-air.

Computers:

Paraglider started as a high-risk equipment back in the 80s. They used to have glide ratios of 2:1 which allowed steering away from steeper mountains, but that's it. Today we can actually fly with glide ratios of 8:1 and up, but those where only possible with extensive computer simulations. Another aspect is safety: Today's entry-level paraglider can sort out all but the most severe disturbances without human intervention, thanks to extensive simulation. Back in the days catching a harsh turbulence meant deploying your reserve chute, if you where lucky enough to have one.
So simply put, a paraglider looks simple, but is fairly complicated. We do annual checks where we measure the individual ropes' length, knowing that a single inch of stretch could alter the flight characteristics to a fatal degree - it's easy to see that it was troublesome to find these perfect proportions.

Is it really true that computer modelling is extensively applied to the design of modern paragliders?  That would seem to be extremely challenging due to the extreme flexibility of the aircraft; my understanding that it is not much applied to the design of modern hang gliders.  Could be the basis for another ASE question; if you have some knowledge on this you could ask and self-answer.

@quietflyer As an active paraglider pilot I followed the development of ever better wings over the years, and remember that all companies attributed that to "better software", but struggle to find solid sources right now. You mention that this would be challenging - it surely is - but it would be even more so to do it with bare trial&error.

@quietflyer yes it's very true. They use very powerful computers and very clever people in all modern paraglider wing design.


text:
Are there technical means to enforce aircraft speed limits as well as regulatory means?
Do any aircraft have a device which can be pre-set or can be controlled from the ground in order to prevent the aircraft from exceeding a speed limit?
These could be similar to what they have for buses in many places so the bus driver can't go beyond a speed limit.

I have seen them on London buses and some places in India as well.
https://en.wikipedia.org/wiki/Speed_limiter

I think that such a device would be incredibly dangerous to have on a plane. Airplanes fly by going fast, and some situations might require a higher speed to remain airborne.

Anything that could control an aircraft from outside would not be accepted (unless we talk about proposals in the event of hijack etc).  The only people who have the full picture and the context required to manipulate the controls are inside the aircraft.

@airsick The article isn't perfect but I'm not sure why you're insistent on being argumentative. The EU does insist on physical speed limiters being implemented on many vehicles and they're now extremely common on busses, HGVs and vans.

What problem would such a device would solve? If there's no need for the device, there's no reason it would exist.

For what it's worth, many trucks, busses, etc. have speed limiters in them even here in the US.  Mostly commercial vehicles that are trying to limit liability.

Some aircraft have flight envelope protections which, among other things, aim to prevent the aircraft exceeding a predefined maximum airspeed.

High Speed Protection:
Prevents exceeding VMO or MMO by introducing a pitch up load factor demand.
The pilot can NOT override the pitch up command.

- AirbusDriver.net
Note that these are structural safety limits, people on the ground cannot use this feature to impose arbitrary speed limits by geographic area, time of day etc.

What does it do if pitching up sharply enough to keep the plane from exceeding VMO/MMO would itself cause the aircraft to exceed structural safety limits?

Short answer, no.
Speed control is somewhat more complicated in an aircraft. Surely it would be technically possible to install a device that would limit engine power, but it cannot control speed.
Once airborne, you can always increase speed by pitching the nose down or decrease speed by pitching up. Similar effect can alter speed when flying through strong up- or downdrafts while maintaining level flight.

Just pointing out that thrust is not the only way to control airspeed.

I disagree. It is possible to set a target speed or a speed limit, but you cannot prevent the aircraft from exceeding it in some situations.

You can do the same as in the alleged bus: prevent the pilot to intentionally move faster than a limit. Now neither the aircraft or the bus can be prevented to exceed this speed in some situation where the force is not under control, e.g. sudden change in slope for bus (we see that in our car speed limiter), or sudden change of angle of attack in an aircraft. But overall this works (and I suppose that's the intent of the question to understand this) if we allow a delay between the event and the feedback.

I agree fully on your last comment. The keyword is "intentionally". However I understood the question more literally, and therefore my answer is no, exceeding a speed limit cannot be prevented by a device.


text:
What's the purpose of this red marking on the fuselage of a Boeing 737?
I was watching some aircraft today and noticed a red marking on two of them.

What is that? I've seen that marking on many other aircraft.

@SMSvonderTann Please go here and suggest "737" as a synonym for that tag. It will link them together.

@TomMcW Can't do it because "737" does not have enough score.

@SMSvonderTann is correct. The marking is there for warning ground personnel about engine hazrd. In this photo (of a 737) you can see 'INGESTION HAZARD' written above the line.

Image from airliners.net
In this image it is possible to zoom in to see it much more clearly. Note that this marking is not found in all 737s. The danger area is marked with two lines- one in forward fuselage and another in the engine, as can be seen in @mins photo, which I've put below.

Image from airlinereporter.com
Also, as @mins notes, the lines in 747 are for equipment positioning (to prevent damage to fairing, looks like)

as discussed in meta, please consider giving a useful source to the images: http://meta.aviation.stackexchange.com/a/2889/1467

They are the markings for danger zone around the engines telling people to stay out of that area. If you notice, the engines also have that marking as well. They correspond to the side of the aircraft in the danger zone image:

Image from boeing.com

Danger when the engine is running? You can go back as far as the red line an not get sucked in? At what engine speed?

This answer establishes that the engines have reason for such a marking, but but establishes nothing regarding the forward fuselage.

DO YOU HAVE A SOURCE FOR THIS?  I ask because your track record for making things up that have absolutely no basis in reality is out there (metal rivets in a radome), and I fly these airplanes & have never seen any such marking as shown used for any such purpose as this. IS THIS YOUR OWN IDEA, OR CAN YOU SUPPORT IT?

@RalphJ I saw it on an older SE question that I can't find. If you so want it I can delete this until I find it.

This Boeing article (which incidentally appears to be where this answer's images were taken) gives a safety zone of up to a 14 ft radius from the center of the intake. The article also documents factory engine markings and suggested pavement safety zone markings. However, it gives no reference whatever to fuselage markings.

SMSvonderTann: to avoid future discussion about sources, it would be beneficial to provide the links to the pages from which you take the images, not generics "boeing.com". As the comment from @JonathanWalters shows.


text:
How quickly should heading changes be accomplished?
While airborne if there needs a change in heading, then what is the rate of change of heading with respect to time?
Like, I want to change my heading from x degrees to y degrees, where the difference is 45 degrees, do we have some specific time limit in which we have to get this change and do we have some regulation/standard for specific rate of heading change?

According to Wikipedia, a "Standard Rate Turn" is three degrees of heading change per second.
The bank angle required to achieve this depends on your true airspeed; at a low TAS a standard rate turn is easily achievable with normal, comfortable bank angles (say, 20°, plus or minus).  At high true airspeeds, the bank required becomes excessive, and "half standard rate" is used if anything is used, but at least in airline flying it's not particularly common to use either.  We just reference an angle of bank — but what that gives you in degrees per second is variable (as a function of TAS again).
The upper limit of what an airliner (or pretty much any IFR aircraft) is going to give the controller in terms of turn rate is whatever you get with a 30° bank.  And if that produces less than standard rate or whatever, then so be it.  At high altitude (say 35,000' and above), even 30° can be on the edge of problematic (mainly, with regard to wing loading and Mach buffet), and controllers are used to rates of turn that are less than what you'd see down low.
But if you're looking for a ballpark, 3 degrees per second at low airspeeds (say, under 200 knots), then 2 degrees per second up to maybe 300 knots, and 1 degree per second above that.  Ballpark, but reasonable numbers.

At high altitudes turns are often either half standard rate or bank limited to 15 degrees.

3DPS = "2 minute turn".

The Turn Indicator (shown below) has a mark that shows a standard rate turn which takes 2 minutes to go all the way around. The heading indicator (on the right below) can be used to determine how far you have turned or how close you are to your desired heading:

There are no "regulations" about turns. A pilot will typically use standard good pilotage skills to make a turn of a type which is needed for the situation. In small planes, the pilots are taught to make crisp, decisive turns to get on course as soon as possible. Large planes with passengers tend to make very gentle turns in order to keep the passengers as level and undisturbed as possible.
Note that a pilot can make a controlled turn onto a heading with just the turn coordinator. In a standard rate turn you do 3 degrees per second, so if you need to turn, say, 15 degrees, you adopt a standard rate turn and count 5 seconds. This technique can be used to escape after flying accidentally into a violent storm. The pilot makes a standard rate turn (15 degrees) then counts 1 minute (60 seconds). This will cause the plane to reverse direction and hopefully lead out of the storm.


text:
Do pressure altimeters take into account differences in temperature?
I work as ATC in Korea and recently, I have some question about winter altimeter setting.
In my facility, we use winter altimeter adjustment because the aircraft's pressure altitude is lower than true altitude in winter. I know this is due to air density difference in winter and summer.
My question is, can the altimeter setting (inch or QNH) include this adjustment? I know that the altimeter setting which ATC gives to the pilot changes every time according to the ground QNH condition. If the altimeter setting can include the adjustments, it would be convenient to ATC and pilot.

I thought transponders always send the pressure altitude (i.e. altimeter set to 29.92)?

I didn't understand your question completely, can you give some specific examples of the QNH or other altimeter settings? I think that would help to make it clearer

This is a great question. I think the answer has to do with the fact that the QNH is meaningful for more than just altimeter settings, so is not adjusted to reflect cold weather conditions. This cold-weather effect also impacts instrument approach minimums.

Related

Related

What adjustment for temperature? QNH (whether expressed in inHg or hPa; those are just units) is whatever will make the altimeter read correct value at field elevation.

The most recent edit by Steve V changes the question to be regarding the altimeter itself. I had understood this question to be about the QNH... how are others seeing the same thing as Steve (see edit history for old version)? I think the question has changed substantially.

Also related though not a duplicate -- how required corrections are indicated on an approach chart.

@mins "Winter" is a brand, not a description. They work in all 4 seasons

You can't just adjust the QNH setting to compensate. The temperature adjustment is not linear. The error increases with altitude, so when the altimeter is set for QNH the error at field level is zero. But it increases the higher you go. It's not just everything being X number of feet higher or lower.
In your question you say "the aircraft's pressure altitude is lower than true altitude in winter." That's backwards. The actual altitude is actually lower than what is indicated by the pressure alt. This can run the risk of colliding with terrain if the pilot doesn't adjust. In fact, exceptionally hot weather causes an error also, but in the opposite direction. The reason this doesn't need to be compensated is because the actual altitude will be higher than indicated so it doesn't create a danger of hitting terrain.
From answers to another question an aircraft with an FMS can be set to automatically adjust.
The problem seems to be in who's responsible for making the adjustment (ATC or pilot) and being clear on whether the altitude announced is adjusted or not. According to the answer from wbeard52, in the US the FAA has established a list of airports with cold weather procedures that require the pilot to do the compensation and announce the corrected alt to the controller. According to Sami's answer, in the EU the controller makes the corrections under radar coverage, and the pilot makes the correction without. It would appear from your question that in Korea they have decided to have you make the corrections as controller.

@mins Ah. Was kinda wondering how the heck that would work


text:
What device in western countries allows ATC to get clearances or search flight plans automatically?
In Korea, there is a device called 'Flight data terminal' which enable air traffic controller to get clearance or search certain flight plans automatically. But I found that there is no 'FDT' in western countries. I believe there must be some devices whose functions are similar to FDT of Korea, so I want to know the name of that device.

Could you please explain in more detail what this FDT system does? Without knowing the system, it is impossible to find other systems that work the same.

Thanks. The main function of FDT is transfering flight plan to ACC and getting clearance. We don't need to call to get ATC clearance. By clicking, we can get clearacne, serach flight plans, and view some details of aircraft like SSR, altitude, destination, and so on.

The European equivalent would be the Flight Data Processing System (FDPS). The FDPS receives flight plans over the Online data interchange (OLDI) / ATS Message Handling System (AMHS) from the Integrated Initial Flight Plan Processing System (IFPS) which is operated by EUROCONTROL. In addition local flight plan can be added to the FDPS.
Aircraft operators file their flight plan with the IFPS, which checks the flight plan for correctness (syntax errors, route errors, etc) and determines whether the plan is acceptable to the air traffic service providers. If necessary, plans are rejected or modified. Once the plan is acceptable, it is distributed to all Air Traffic Control units (ATCU) along the route.
In the ATCU, the FDPS will receive the flight plan and store it in its flight plan database.
It must be noted that the FDPS is much more than just a flight plan database. It is at the heart of the air traffic control unit. It correlates flight plan data with actual radar tracks, updates flight plan with inputs from air traffic controllers and it supports local flow management functions, conflict detection systems and much more.

Without knowing the Korean system, it is impossible to tell exactly what you are looking for. However, based on your minimal description, it sounds like you might be looking for systems like CPDLC or other data link systems. These are not automated systems per se, but simply systems that allow digital, text-based communication between ATC and pilots, instead of VHF-based voice communication.
With regards to automation, you might want to look into tools such as medium term conflict detection (MTCD) and arrival management tools (AMAN).
Automatic exchange of fight plan data is done, in Europe at least, via online data interchange (OLDI).


text:
Are aircraft with counter-rotating props or Down Between Engines less likely to develop asymmetric icing?
It is suggested by some (and somewhat substantiated) that twin-engine propeller aircraft which do not have counter-rotating props (all modern regional turboprops are this way) can develop asymmetric wing icing due to the airflow pattern generated by the propellers:
 (from the FAA FFFSCR report, via the linked source)
Has it been demonstrated that twins with counter-rotating props (are any such twins rated for FIKI?) are immune to this asymmetric icing phenomenon?  What about quad-turboprop planes such as the A400M, with its Down Between Engines configuration?

Ice accretion is a very complex phenomenon on airplanes that depends on several factors. One of the main factor is the composition of the cloud that the airplane is flying and the exposure time.
Another significant effect are the conditions on the imprigment surface on the airplane, which depends of the flow path and the pressure over the surface.
So, having the properlers rotating in the same direction means that the propellers are providing energy in a different direction. Also that will mean that, for optimizing the airplane, the fuselage will be slightly assymetric (very difficult to notice).
So, to the first statement you are mentioning, I can confirm you that the imprigment areas affected by this assymetric effect will develop different ice shapes.
What I can tell you is that FAA and EASA employ real experts for airplane certification and they are concious of this effect, and I can tell you that any doubt they have about aircraft performances will request the manufacturer to demonstrate in flight.
My answer to your question is that those airplanes are not protected in the sense that they will develop assymetric iced conditions but are strongly protected in the sense that this condition is considered and very likely tested with airtificial iced shapes, so the airplanes are really safe. So globally, yes, the airplane is "protected" by design.
For your questions about 4-engined airplane, the answer is the same but having more complexity due to the number of engines.
I can tell you as well that in iced conditions, due to the complexity of the phenomena and the prediction and simulation difficulties, there is a lot of conservatism in the way manufacturers design the airplane and significant cost in airplane testing. FAA and EASA are really doing a good job.

"Also that will mean that, for optimizing the airplane, the fuselage will be slightly assymetric (very difficult to notice)."
Do you have a reference for this statement?

We are talking about small diferences in design sometimes located in the fairing of the wing-fuselage. Almost irrelevant to notice and sometimes is not implemented due to the cost of having 2 difference pieces. I don't have a reference I can share.

@TrebiaProject. the digression into certification requirements isn't really needed *chuckles*

Puff... I've got kind of emotional sorry :):)


text:
What are the curved centerlines at taxiway intersections for?
At all airports where I've looked out the window during taxi, the taxiway intersections have been painted with nice circular arcs connecting (the centerline of) each entry to the intersection with each exit.
I've always assumed these lines were there to guide planes through the turns -- that is, if the nose wheel follows the curved line exactly then you're guaranteed that there'll be pavement under the path the main gear will follow, and there will be sufficient wingtip clearances for the aircraft types allowed to use the intersection, etc.
However, in this question it comes to light that pilots can't even see the curved centerlines while they're turning and rely on spatial intuition instead. Very well, that seems to work fine in practice.
Which leads to the question: Who are the curved centerlines for, then? What would go wrong if they weren't there and instead just the straight centerlines of the intersecting taxiways continued through?
Possibly the pilots of smaller aircraft than the intersection is built for (say, a Dash-8 in an intersection that can take A380s?) can see the curved lines ahead of them while they turn. But in that case they should have plenty of room to do the turn by intuition too, right?

The line along taxiway centreline is meant to provide guidance before initiating the turn. You're right that in a large aircraft they would lose sight of the centreline when they're over it, but it gives the pilot an idea of where they need to keep the nose in order to keep the outer main wheels behind them on the taxiway surface.
For example, in Canada, Transport Canada document TP312 (Aerodrome Standards and Recommended Practices) define how you need to place the centreline in order for the outer main wheels to remain at least a certain distance from the taxiway edge. (I believe the United States would have very similar rules; I just don't know the names of the appropriate documents)

Taxiway Curves
3.4.1.6 Recommendation - Changes in direction of taxiways should be as few and small as possible. The radii of the curves should be compatible with the manoeuvring capability and normal taxiing speeds of the aeroplanes for which the taxiway is intended. The design of the curve should be such that, when the cockpit of the aeroplane remains over the taxiway centre line markings, the clearance distance between the outer main wheels of the aeroplane and the edge of the taxiway should not be less than those specified in 3.4.1.3.

Think about when a large aircraft has to make a big turn (>90degrees). If there was no indication on the surface about how to take the turn in a large aircraft, you could inadvertently take it too early and run the main wheels off the pavement.
Here's an example of an aircraft following the centreline with its nose as best as possible in order to keep the main wheels in the right spot.

Your references and explanation match the intuitive idea of the purpose I had before. But I have trouble seeing how the line can serve that purpose when the pilot cannot see it anyway. I suppose it's right that the pilot will know it is too early to turn if he can still see the curve he's supposed to follow, though ...

@HenningMakholm it isn't necessary to see the nose wheel rolling over the line in order for it to provide effective guidance - Taxiing a Cessna is like driving a small car: you can see all the markings pretty well & it's easy to use them for reference. Taxiing a 747 is like driving a cement truck: you can't see the markings close to you, but that's OK because you saw them a few seconds ago, and you can see the ones coming up, so you learn to anticipate and put the wheels where you want them.

These days, more airports are doing offset centerlines in curves (or just more space on the inside of the turn I guess) so that you actually can see them and use them for guidance. Here's KRSW at F1 and Runway 6.
.

While this is factual (from your picture), it doesn't answer the question asked.


text:
Backup plane for checkride: Can it be a different model?
I'm a student pursuing a US Private Pilot License, and recently scheduled my checkride. I've been training in a 1981 Piper Warrior (PA-28-161), but if its annual goes sour I may have to take my club's 1980 Piper Archer (PA-28-181). I have well over §61.109's 40 hours in the Warrior alone, and only ~10 hours in the Archer. I have a separate club checkout and CFI solo endorsement for each, they're pretty similar anyway—they're even both the same ATC type (P28A)—but they aren't the same model.
Here's the catch: Form 8710, the "Airman Certificate and/or Rating Application", asks specifically for

[II.A.2.] Total time in this make/model and/or approved FFS or FTD (Hrs.)

Furthermore, according to the IACRA FAQ ("I'm a Designated Examiner. I noticed a mistake when reviewing the IACRA application...."):

A mistake in the applicant's information cannot be corrected after it has been signed by the Recommending Instructor.

My prospective DPE explicitly told me that the backup plane was fine, and I can't find any Part 61 regulations that are specific to experience in one make/model aside from adding an experimental aircraft as part §61.63(h)(1), which is what I assume the box is really for. Of course I'd rather test in my more-experienced plane, but I'm asking specifically for regulations here if Plan A falls through.
Are there any regulations or headaches that I would encounter if I need to change the tail number or model number of my checkride plane and listed hours therein, potentially long after IACRA submission?

This is actually an interesting question - From an ATC perspective these are "the same plane" (the type code is P28A), but from an IACRA application standpoint they're categorized as PA-28-140, PA-28-150, PA-28-160, PA-28-180, PA-28-161, PA-28-181...

You probably already know this, and it doesn't answer your question, but note that the examiner will expect you to demonstrate understanding of every system in the airplane. I have heard of similar situations for instrument checkrides where a student switched to a different plane at the last minute, the plane had a backup #2 GPS or stormscope or something that their usual training plane didn't have, they didn't know how to use it, and they failed the checkride. (Even though the device wasn't required for IFR.) So make sure you know all the equipment/systems in your "Plan B" airplane!

I came to the same conclusion as you, I don't see anything keeping you from going. You meet the requirements of 61.107 and 61.109, you have recent experience in both aircraft, and as far as make and model, they're both PA-28's. That's like going from a 172B to a 172S: it's the same make and model, Piper just makes it a little less obvious with their nickname and number combinations. If you had trained in a Warrior 160 and an Arrow, it would be a bit different, but only as far as requiring an additional endorsement.

Thanks. I agree it's no show-stopper—I can't be the first to encounter this, can I?—but I do wish it were rather more explicit just what to do here. FWIW my CFI did give me a second solo endorsement for the second model, just to be sure.

I think I agree with your answer, but there can be very significant differences between e.g. a 172x and a 172y (e.g., glass/G1000 vs. traditional panel). Differences big enough to seem significant with respect to private pilot examinations. Same with PA-28-xxx (consider the Dakota, PA-28-236, which has a constant-speed prop and is "high performance", vs. the lower-HP fixed-pitch Warrior/Archers). (Interesting side note, even though the model is PA-28-236, its ATC type code is P28B... hmm.)

@dvnrrs My inclination is somewhat the same as yours: go by the ATC type code (i.e. consider all the P28A PA-28s to be the same, all the P28B PA-28s to be the same, all the P28R PA-28s to be the same, etc.) for checkride purposes. (For solo endorsement purposes I was endorsed separately for each model I flew (PA28-140, PA28-161, and PA28-180) when I was training like Jeff was - not sure if that's strictly required, but it's certainly not a bad idea.) You could Garminize a 1960 Cessna 172, but it would still be a 1960 Cessna 172 :-)

Epilogue: I confirmed with my phase check CFI (and former DPE) that it's no big deal. She noted that the airplane designation "can be lined through" the day of the test.
In any case, the PPL doesn't mandate experience in a single type of aircraft, and anything one can fly in a PPL checkride requires instruction and an endorsement, so presumably the type can be adjusted even more significantly. (Whether it's a good idea is another matter entirely, but...)